The present invention relates broadly to a system for transferring the response of an electrically controllably mechanism from one control system to another without an abrupt change in the mechanism in accomplishing such transfer. This is particularly desirable in transferring the operation of the governor or control valves of the steam turbine between a single and sequential mode of operation.
In the single or full arc mode of operation, the valves are operated identically to control the flow of steam through a full arc of inlet nozzles to the turbine blading. In this mode, all the valves respond equally to a change in the flow demand or control signal. In the sequential or partial arc mode of operation, the valves are operated sequentially in a predetermined pattern to control the flow of steam through a partial arc of inlet nozzles to the turbine blading. In this mode, one or more valves respond differently to a change in the total flow demand or control signal.
Therefore, the valve position control signal for each of the valves when operating in the sequential mode is different than it is for each of the valves when operating in the single valve mode. To suddenly change operating modes, by switching abruptly between single and sequential control signals, could create an abrupt positional valve change that would introduce transients in the system. Such transients could possibly cause damage to the valves and/or the turbine blading.
Thus, it is desirable that a transfer from either the sequential mode or the single valve mode to the other be gradual; that is, that the value of the applied signal during the transfer gradually changes from the value of the one signal to the value of the other signal. Also, it is desirable for the mechanism, such as the valve control mechanism, for example to respond quickly to a change in either one of the control signals during such transfer without an abrupt change in the signal applied to the mechanism. A change in one of the control signals should affect the applied signal during such transfer depending on the percentage of transfer completion. Also, it is desirable that such transfer system be capable of effecting such a transfer effectively over an extended period of time, such as up to eight minutes, for example.
In U.S. Pat. Nos. 3,367,319 and 3,740,588 issued to Stratton et al, there is proposed a dual mode control system and method for changing between a single and sequential mode of valve operation. The proposed system utilizes what is termed time ratio switching for changing from one mode to another. The method used in the proposed system is to alternately switch between the two control signals at a given rate. This is accomplished by generating saw-tooth shaped pulses at a given or constant rate. A ramping voltage signal, which gradually increases throughout the transfer period, is compared with each saw-tooth waveform or pulse to produce at such given rate, a first series of output pulses that are successively greater in width, and a second series of output pulses that are successively narrower in width, until each expanding pulse is equal in duration to the repetition rate of the saw-tooth generated pulses and the narrowing pulses disappear. The width or duty cycle of each of the pulses is varied by a comparison circuit that chops each of the saw-tooth pulses at a different height in accordance with the progressively increasing ramp voltage.
A transfer system, which modulates the frequency of the generated pulses that are applied to the control mechanism, in contrast to a system where the generated pulses are varied in width or duty cycle to affect a transfer, can produce the desired results with the use of reasonable resistor and capacitor filter valves. The higher the frequency, the lesser are the component values of the filtering mechanism.
Also, a frequency modulation transfer system does not depend for reliability on a time-dependent constant-increasing voltage and its accompanying comparison circuit, which is difficult to control particularly during an extended transfer period.